The modern production of cultured pearls generally encompasses three basic techniques. First, spherical pearls of all sizes are produced by the well-known tissue-graft technique using a pearl nucleus, in various marine species of the pearl oyster as well as the fresh water mussel. Second, large (10-22 mm diameter) semispherical pearls are produced using the so-called "blister pearl" technique, again in pearl oysters and fresh water mussels. Third, irregular or baroque pearls are produced in fresh water mussels, using a multiple tissue-graft process without a pearl nucleus.
The production of cultured pearls in oysters, however, is limited geographically to a significant extent by the temperature of the sea water and other environmental conditions. A large number of potential growing areas for pearl oysters are excluded for these reasons, including the coastal waters of the eastern Pacific Ocean off the United States and Canada. However, several species of abalone, which have the ability to culture gem-quality pearls under selected conditions, are adapted for the cooler waters. Heretofore, abalone pearls have been cultured primarily by the Japanese. The available literature includes several papers which have been published in Japanese concerning the use of abalone to culture pearls. One of the earliest and most significant papers in the field is an article by Uno entitled "Pearl Culture Using Abalone," published in The Aquaculture in 1957. In this article, the production of both round and semispherical abalone pearls is discussed.
Abalone pearls are highly prized commercially, because their nacre (the deposited material) features the multi-hued colors so admired in abalone shells. The Uno article describes a general procedure for culturing blister pearls in abalone, including the initial step of abrading a hole using emery powder through the shell of the abalone, for the insertion of a nucleus element inside the shell. Next, a semispherical pearl nucleus is attached to a thin sheet of Bakelite plastic and is inserted through the abraded hole to a position against the abalone's mantle tissue. The thin plastic sheet is then cemented to the outside of the shell, holding the nucleus in place. The shape of the nucleus will determine the shape of the pearl.
Some success has been achieved in the production of semispherical pearls in abalone. Such pearls will typically range from 10-22 mm in diameter. However, attempts to produce spherical pearls in abalone using Uno's techniques have met with very little success, resulting in distorted pearl shapes and also a high incidence of infection, which in turn causes nucleus rejection and an increase in mortality of the animal. Hence, spherical pearls, as far as is known, are not currently being cultured in abalone.
U.S. Pat. No. 3,871,333, which issued to Gotoh on Mar. 18, 1975, relates to a method of producing cultured pearls in abalone which includes the steps of perforating a hole in the shell of the abalone overlying its reproductive organ and introducing a pearl nucleus assembly into the interior of the abalone through the hole. The Gotoh pearl nucleus assembly comprises a thin wire or pin, a round bead secured to one end of the pin which is positionable adjacent the reproductive organ of the abalone, an inner disk for substantially occluding the shell hole, and a larger outer disk which may be affixed to the abalone shell with a water insoluble bonding agent to maintain the assembly in place after implantation.
Historically, it has proven quite difficult, cumbersome, and time-consuming to insert pearl nuclei into abalone shells and the like and then glue them in place, such as is taught by Gotoh in the '333 patent. The primary drawback of this technique is that the bonding agent alone may not be sufficient to maintain the entire pearl nucleus assembly securely in place. Often the host abalone uses its powerful foot muscle to vigorously exert pressure against the inserted pearl nucleus which may dislodge the nucleus from the implanted position before conventional cements or glues have a chance to fully set. This is particularly the case if the abalone shell is not dry and free of surface films before the outer disk is bonded to the abalone shell. Cleaning and drying of abalone shells is simply not practical for large scale commercial applications where time is of the essence.
Further, with conventional techniques, the mortality rate for abalone is quite high, due to several factors, including tissue abrasion, thermal exposure and desiccation. The rate of infection is also relatively high, primarily due to tissue trauma resulting from abrading a hole through the shell of the abalone with emery powder or the like.
Accordingly, there is room for significant improvement in existing techniques for implanting pearl nuclei within molluska such as abalone for the culturing of pearls. In particular, it is desired that the present process be greatly reduced in time, increased in reliability, and that the mortality and rate of infection for the animal be significantly reduced.